1. Field of the Invention
The present invention relates to an active element having a heterojunction bipolar transistor and a switching circuit device, and particularly relates to a temperature-compensation active element and a switching circuit device.
2. Background of the Related Art
A heterojunction bipolar transistor (hereinafter referred to as HBT) has a higher emitter efficiency and a higher current amplification factor hFE as compared with a general homojunction bipolar transistor, and thereby it is possible to largely increase a base impurity density and equalize a transistor operation over an entirety of the base. As a result, as compared with a GaAs MESFET (Metal Semiconductor Field Effect Transistor), a GaAs JFET (Junction FET), and a HEMT (High Electron Mobility Transistor), a current density is high and an on-resistance is low to offer excellent results in an efficiency, a gain characteristic and a distortion characteristic.
A small-sized high frequency switching element with a high-efficiency is indispensable for mobile communication equipment such as a cellular phone or the like. Then, as illustrated in FIGS. 26A and 26B, there has been known a switching circuit using a bidirectional HBT as a switching element.
FIGS. 26A and 26B illustrate an example of a switching circuit using the HBT as the switching element. FIG. 26A is a circuit diagram, and FIG. 26B is a cross-sectional view illustrating a structure of the HBT.
As illustrated in FIG. 26A, this circuit includes a first HBT 320 where an emitter is connected to an antenna ANT and a second HBT 321 where a collector is connected to the antenna ANT, and a collector of the first HBT 320 is connected to a transmitting circuit Tx. Moreover, an emitter of the second HBT 321 is connected to a receiving circuit Rx, bases of the HBTs 320 and 321 are connected to a transmitting control terminal CtrlTx and a receiving control terminal CtrlRx through a resistor 322, respectively.
As illustrated in FIG. 26B, an n type GaAs subcollector layer 311 is formed on a semi-insulating GaAs substrate 310, an n type AlGaAs collector layer 312, a p type GaAs base layer 313, an n type AlGaAs emitter layer 314, and an n type GaAs emitter contact layer 315 are grown on the subcollector layer 311 in a mesa shape.
On a surface of the subcollector layer 311, a collector electrode 316 is placed at a position where the collector layer 312 is interposed therebetween. On a surface of the base layer 313, a base electrode 317 is placed at a position where the emitter layer 314 is interposed therebetween. An emitter electrode 318 is arranged on an upper portion of the emitter contact layer 315. The HBT of a minimum unit illustrated in the FIG. 26B is used as a unit element 320′ (321′) and these elements are connected in parallel to form the first HBT 320 (second HBT 321) as the active element. This technology is described for instance in Japanese Patent Application Publication No. 2000-260782
The emitter electrode 318, the base electrode 317 and the collector electrode 316 of the HBT are formed in a comb-tooth shape. Then, the structure shown in FIG. 26B is used as one unit element, and a plurality of unit elements are connected in parallel to form an active element such as a switching element.
In the HBT, since a current between the base and the emitter has a positive temperature coefficient, a collector current also has a positive temperature coefficient. Accordingly, when a base current is increased to improve a current density, the current concentrates on one unit element of the plurality of unit elements of the HBT connected in parallel to cause a secondary breakdown and easily lead to destruction of the HBT.
Conventionally, there was a problem in which the current density could not be sufficiently improved to avoid occurrence of a reliability problem.
Moreover, in general, measures in which an emitter ballast resistor or a base ballast resistor is inserted into the comb-tooth like unit element 320′ of the HBT 320 are always taken in order to solve this problem. However, the insertion of the emitter ballast resistor or the base ballast resistor causes a new problem in which a high frequency characteristic is degraded accordingly.